1. Field of the Invention
This invention relates to a new process for the production of oligomeric polyisocyanates containing uretdione and/or isocyanurate structural units by the dimerization and/or trimerization of a portion of the isocyanate groups in organic polyisocyanates using tertiary phosphines or peralkylated phosphorus acid triamides as dimerization and/or trimerization catalyst, followed by termination of the dimerization and/or trimerization reaction by the addition of catalyst poison, sulfonyl isocyanates, optionally in combination with organic acid chlorides, being used as the catalyst poison, and to the use of the products obtained by this process in the production of polyurethane plastics.
2. Description of the Prior Art
The production of isocyanato-uretdiones or isocyanato-isocyanurates or mixtures thereof by the dimerization and/or trimerization of a portion of the isocyanate groups in organic polyisocyanates using aliphatic, araliphatic or mixed aliphatic-aromatic tertiary phosphines or peralkylated phosphorus acid triamides as catalyst is already known. The question of whether dimerization products predominantly containing uretdione groups or trimerization products predominantly containing isocyanurate groups are formed in this reaction depends to a large extent upon the degree of conversion and upon the temperature profile (cf. A Farkas and G. A. Mills, Adv. Catal. 13 (1962) pages 393 et seq).
Generally, it may be said that with increasing temperature and increasing reaction time, the reaction proceeds via the uretdione preliminary stage to the isocyanurate trimers predominantly containing one isocyanurate ring and, ultimately, to isocyanurate polyisocyanates which, in addition to monoisocyanurates, also contain homologs with more than one isocyanurate ring. Both the predominantly pure dimerization products and mixtures thereof with the trimerization products and also the predominantly pure trimerization products represent valuable starting materials for the production of polyurethane plastics, the degree of dimerization or trimerization being adapted to the particular application envisaged in the production of oligomeric polyisocyanates. In either case, however, it is essential, if production is to be carried out reproducibly on an industrial scale, to terminate the dimerization and/or trimerization reaction precisely and quickly at a predetermined point.
In order cleanly to terminate the reaction at the desired conversion level, compounds which are intended to neutralize the effect of the catalyst are frequently added to the reaction mixture.
According to DE-OS No. 1,670,667 and to DE-OS No. 1,670,720, these compounds are preferably alkylating agents such as dimethyl sulfate, methyl iodide or toluene sulfonic acid esters, and acylating agents such as carboxylic acid chlorides and carbamic acid chlorides. However, these compounds are attended by the disadvantage that they do not neutralize the effect of the catalyst instantly and completely. Instead reheating is generally necessary (thus, an increase in temperature to 80.degree. C. is proposed in DE-OS No. 1,670,720), with the result that the polymerization reaction cannot be cleanly terminated.
According to DE-AS No. 1,954,093, sulfur is suitable for deactivating tertiary phosphine catalysts. Although sulfur has the advantage of deactivating the tertiary phosphine catalysts spontaneously and quickly, the phosphine sulfide formed is generally a readily volatile compound. Thus, when the unreacted starting polyisocyanate is removed from the reaction mixture by distillation, the phosphine sulfide distills off with it and contaminates the distillate. Since the distillate is normally recycled, the amount of phosphine sulfide accumulates to an increasing extent.
In the processes according to U.S. Pat. No. 3,290,288, DE-OS No. 3,030,513 and DE-OS No. 3,227,779, peralkylated phosphorus acid triamides, such as tris-(dimethylamino)-phosphine, are preferably used as catalysts. At the same time, it is proposed to subject the reaction product to distillation on reaching the required conversion level without deactivating the catalyst. Although the catalysts mentioned accelerate above all dimerization, but not trimerization, it is nevertheless possible when this procedure is adopted for uncontrollable quantities of trimers to be formed from the uretdiones and excess starting polyisocyanates due to the high distillation temperature. This is why DE-OS No. 3,030,513 and DE-OS No. 3,227,779 recommend using only sterically hindered (cyclo) aliphatic starting diisocyanates. Thus, it must be assumed that the process according to those prior publications only enables dimeric diisocyanates to be reproducibly obtained where special starting diisocyanates are used. In addition, it is important to point out that the distillate which contains the active catalyst cannot be subjected to prolonged intermediate storage before it is reused because, in that case, it would undergo polymerization throughout and would thus become unusable.
Accordingly, the object of the invention is to provide a new process for dimerizing and/or trimerizing organic polyisocyanates using the state-of-the-art catalysts mentioned in the foregoing, which enables the reaction to be stopped satisfactorily and immediately at the particular conversion level required without being attended by any of the disadvantages of the prior art cited above.
This object is achieved by the process according to the invention which is described in detail hereinafter.